It has heretofore been known that the thermal stability and barrier properties of oriented blow molded containers of poly(ethylene) terephthalate are significantly increased by heat setting. Typical processes for heat setting are shown in U.S. Pat. Nos. 4,476,170, 4,512,948 and 4,522,779.
In U.S. Pat. Nos. 4,476,170 and 4,512,948, there is disclosed an article and a process of making an oriented and heat set blow molded container of poly(ethylene) terephthalate. In the process, a preform preheated to a temperature suitable for orientation is biaxially stretched in a blow mold and then while the hollow container is still in contact with the blow mold walls, the article is raised to a higher heat setting temperature preferably in the range of 200.degree.-250.degree. C. (except for the neck) thus heat setting the container, and while the container is still at a shrinkage resisting pressure exceeding atmospheric, it is cooled in the same mold to a temperature at which it maintains its shape when not pressurized but not below 100.degree. C. It is also particularly disclosed that this cooling step can be done in the air outside the mold while maintaining internal pressure. According to these patents, when the heat setting temperature of the hot mold ranges from 220.degree.-250.degree. C. and the quenching temperature is not below 100.degree. C., higher onset-of-shrinkage temperatures are obtained.
In U.S. Pat. 4,522,779, there are disclosed improved plastic containers and a process for their production. In the first embodiment, a container is blow molded in a first hot blow mold, then reblown to a larger size in a second cold mold of larger volume than the first hot mold. Such containers are stated as having improved physical properties, particularly very high hoop yield stresses. However, the utilization of a larger volume cold mold substantially reduces the thermal stability. In the second embodiment, a container is blow molded in a hot blow mold, then reblown to a larger size in a second hot blow mold where it is blown to the confines of the second mold and the container is then removed from the second hot mold and transferred to a third cold mold and cooled to room temperature while maintaining internal pressure. In a further embodiment, the container is blow molded in a first hot mold, reblown in a second hot mold, and thereafter the second mold is cooled to cool the container.
U.S. Pat. 4,385,089 (British Patent Specification No. 1,604,203) is directed to heat set biaxially oriented hollow articles and states that the preform or parison should be heated at least to biaxially oriented temperature and maintained in close contact with a hot mold which is at a temperature of up to 40.degree. C. above the minimum oriented temperature. In one embodiment, the resultant molded hollow article is moderately cooled causing a temperature drop of 10.degree.-30.degree. C. by introducing cooling vapor or mist into the hollow article, interrupting the cooling vapor, and opening the mold. In another embodiment, the heat set article is allowed to shrink freely and then reblown in the same hot mold or in a separate cooled mold. The patent calls for a heat setting temperature of 40.degree. C. above the orientation temperature limits thermal stability and barrier properties from heat setting.
According to this patent, the temperature of the hot mold should be maintained between 30.degree. and 50.degree. C. above the minimum orientation temperature of the plastic material. Otherwise, it is stated there are numerous disadvantages including lowering of the production rate, the danger of the appearance of major distortion and major shrinkage on mold release, the disadvantage inherent in heating metal molds to very high temperatures and keeping them at such temperatures, and the danger of crystallization which would cause a loss of transparency. Further, in accordance with this prior patent, excessive shrinkage is to be avoided and generally the temperature drop of 10.degree. to 30.degree. C. should be made. Accordingly, such a method precludes obtaining a degree of heat setting which would produce thermal stability at higher temperatures as may be required in filling the container with various products. In addition, such a method will preclude obtaining the higher degrees of crystallinity and resultant high barrier properties which are required for some products.
U.S. Pat. No. 4,039,641 discloses a process for producing a heat set biaxially oriented poly(ethylene) terephthalate bottle filled with a liquid wherein a parison is expanded in a mold which is at a temperature in the range of 130.degree. to 220.degree. C. and maintained in contact with the mold by a gas such as pressurized carbon dioxide and the crystallized bottle is cooled by displacing the pressurizing gas with a cooling liquid which is cooled to about 0.degree. to 5.degree. C. The liquid may be liquid to be packaged in the container. Such a method substantially reduces the cycle time because of the need to introduce the liquid and thereafter remove the liquid, in the case where the liquid is not that to be packaged in the container. Moreover, utilizing a liquid within the container to cool the container limits the amount of heat that can be extracted from the container because it has a low coefficient of heat transfer. Furthermore, the low coefficient of heat transfer takes a longer time to extract the heat. In addition, filling the container with a finite amount of liquid equal to the volume of the container limits the amount of heat which can be extracted from the container to the amount of heat that can be transferred to this finite amount of liquid.
U.S. Pat. RE. No. 28,497 discloses a method and apparatus for reducing mold cycle time in a conventional blow molding method wherein a heated parison is expanded by a gas such as gaseous carbon dioxide in a blow mold and thereafter subsequently internally chilled by a liquid carbon dioxide. The article is cooled until it is self-sustaining, the article is vented to atmospheric pressure, the mold is opened, and the article is removed from the mold. The patent does not relate to biaxially oriented articles or heat setting of biaxially oriented articles.
None of the prior art recognizes or teaches that it is necessary to continue the cooling by circulating liquid carbon dioxide after the mold is opened in order to obtain self-sustaining biaxially oriented heat set containers, as in the present invention.
Accordingly, among the objectives of the present invention are to provide a method and apparatus for making partially crystalline, biaxially oriented heat set hollow plastic containers which has a significantly lower cycle time; which permits heat set containers to be made that have complex configurations including oval containers; which involves low capital investment; which is easy to maintain; and which involves the use of lower cost tooling.
In accordance with the invention, the method and apparatus for making a partially crystalline, biaxially oriented heat set hollow plastic container from a hollow parison having an open end and a closed end comprising engaging the open end of a plastic parison which is at a temperature within its molecular orientation temperature range, enclosing the hot parison in a hot mold, the mold being at heat setting temperature, expanding the plastic parison within the hot mold by internal pressurization to induce biaxial orientation of the plastic parison and force the plastic parison into intimate contact and conformance with the hot mold and to maintain contact by such internal pressurization between the mold and the biaxially oriented container for a time sufficient to induce partial crystallization in the biaxially oriented container, exhausting the blow molding fluid while continuously introducing a cooling fluid such as liquid carbon dioxide into the biaxially oriented container and continuously removing the cooling fluid from the container while the hot mold is closed for a period of time, opening the hot mold while continuing to introduce, circulate and remove coolant fluid for a predetermined period of time until the container is cooled sufficiently to prevent significant shrinkage and finally releasing the container.